magnetic anisotropy in spherical fe16n2 core shell nanoparticles determined by torque measurements

Magnetic anisotropy in spherical Fe16N2 core–shellnanoparticles determined by torque measurements Eiji Kita,1,2Kenichi Shibata,1Yuji Sasaki,3Mikio Kishimoto,1 and Hideto Yanagihara1 1Ins

Magnetic anisotropy in spherical Fe16N2 core–shell nanoparticles determined by torque measurements

Eiji Kita, Kenichi Shibata, Yuji Sasaki, Mikio Kishimoto, and Hideto Yanagihara

Citation: AIP Advances 7, 056212 (2017); doi: 10.1063/1.4974276

View online: http://dx.doi.org/10.1063/1.4974276

View Table of Contents: http://aip.scitation.org/toc/adv/7/5

Published by the American Institute of Physics

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Magnetic anisotropy in spherical Fe16N2 core–shell

nanoparticles determined by torque measurements

Eiji Kita,1,2Kenichi Shibata,1Yuji Sasaki,3Mikio Kishimoto,1

and Hideto Yanagihara1

1Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan

2National Institute of Technology Ibaragi College, Hitachinaka, Ibaraki 312-8508, Japan

3Hitachi Maxell, Ltd., 1 Koizumi, Oyamazaki, Kyoto 615-8525, Japan

(Presented 1 November 2016; received 23 September 2016; accepted 25 October 2016;

published online 11 January 2017)

The magnetic anisotropy energy for core–shell α”-Fe16N2 nanoparticles was eval-uated by the rotational hysteresis loss obtained from magnetic torque measure-ments The saturation magnetization of the α”-Fe16N2 core was deduced from volume fractions of α”-Fe16N2 determined by an analysis of a low-temperature Mossbauer spectrum The saturation magnetization and the anisotropy energy were found to be 234 emu/cc and 6.9 Merg/cm3, respectively These values coincide with those of bulk-like single-phase α”-Fe16N2 particles This crys-talline anisotropy is still smaller than the shape anisotropy of the thin films

(2πMs2= 20 Merg/cm3), and a perpendicular magnetic state is not expected

for the thin-film form © 2017 Author(s) All article content, except where

otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ) [http://dx.doi.org/10.1063/1.4974276]

I INTRODUCTION

Fe nitrides have a variety of crystalline structures corresponding to the concentration of nitrogen There are a few ferromagnetic nitrides, and many of these have been studied from the viewpoint of magnetic materials.1,2Early-stage efforts for developing magnetic materials focused on applications for magnetic recording tape media using Fe4N pigments3and recently as a candidate for a spintronics material with a half-metallic electronic structure.4α”-Fe16N2 has a tetragonally distorted structure,

in which one of the <100> axes is elongated and the nitrogen atoms are ordered This substance has been studied by many for its high saturation magnetization;5,6the compound also exhibits a uniaxial magnetic anisotropy7,8due to the distorted structure, showing promise for use in applications such

as high-performance magnetic data recording tapes9 with core–shell type spherical nanoparticles (NPs).10 Recently, new permanent magnet materials that do not require rare elements have been highly sought, and a number of ferromagnetic substances with high transition temperatures have been proposed as potential candidates for the same.11 α”-Fe16N2 is a good candidate, as it has high saturation moments and a large anisotropy constant (Ku) The amplitude of Kuis an important parameter not only for bulk applications12but also for the realization of perpendicular magnetic thin films We have reported that the Ku of the core–shell α”-Fe16N2 NP, estimated from typical torque measurements, is 4.4 x 106erg/cm3.10

Detailed torque measurements were performed to obtain a more precise value of Ku for

α”-Fe16N2 Because the NPs have a core–shell type structure, where a nonmagnetic oxide layer covers the α”-Fe16N2cores, the low-temperature Mossbauer spectra were analyzed to estimate the magnetization of this portion of the α”-Fe16N2cores from the ratio of nitrides and oxides, and Ku was deduced from these values

II EXPERIMENTAL

α”-Fe16N2NPs were prepared using NH3nitrification First, Fe3O4particles with a diameter of

18 nm underwent hydrogen reduction at 450◦C for 4 h Succeeding nitrification was carried out at

2158-3226/2017/7(5)/056212/5 7, 056212-1 © Author(s) 2017

056212-2 Kita et al. AIP Advances 7, 056212 (2017)

temperatures between 150 and 170◦C in an atmosphere of H2and NH3.13Sample characterization was performed using X-ray diffraction (XRD) and transmission electron micrography (TEM).10 Magnetically aligned samples were prepared by drying the organic solvent of particles on the polymer films in a magnetic field parallel to the films

The magnetization and magnetic torque were measured at room temperature (defined in our study

as 300 K) using a SQUID magnetometer, a vibrating sample magnetometer and an automatic torque meter, respectively Torque curves were measured under an external magnetic field of up to 22 kOe and the torques were measured by rotating the external field in the clockwise and counterclockwise directions Rotational hysteresis of the torque was evaluated from the area drawn by the rotation under an external field ranging from 0.5 kOe to 10 kOe M¨ossbauer spectra were recorded at sample temperatures of 300 K and 4.2 K The velocity and isomer shift were calibrated to those of natural Fe foil at room temperature Film samples were used for magnetic and M¨ossbauer measurements Data fitting was carried out using MossWinn 4.0, a commercially available program

III RESULTS AND DISCUSSION

The structural characterization of the obtained α”-Fe16N2 NPs was described in a previous paper.10The particles maintained their spherical shapes between the starting state and post nitrifica-tion The averaged particle size was approximately 20 nm, and the TEM photo9showed a core-shell structure with a crystalline α”-Fe16N2 core and a shell probably composed of amorphous-like iron oxide (hereafter referred as Fe-O) As mentioned below, M¨ossbauer spectroscopy found the outer shell had an Fe3+state, therefore the chemical formula of the outer shell is considered to be Fe2O3 rather than Fe3O4

Figure1 shows the angular dependence of the magnetization curves The maximum coercive force was measured as 3.5 kOe when the angle between the magnetization direction and the external magnetic field was 0◦, and the squareness was found to be 0.89.13The saturation magnetization was calculated as 108 emu/g using the net weight of samples, including non-magnetic components, at room temperature.10

Mossbauer spectra reflect the local environments of Fe, enabling us to estimate the portion of magnetic iron atoms using the area ratio of the spectra The area ratios from spectra recorded at low temperatures are more realistic due to the temperature dependence of recoil-free emission rates.14

We recorded low-temperature spectra at 4.2 K, which are plotted in Fig.2 The numerical fitting was performed under the condition that the spectrum consisted of four magnetically ordered sub spectra and one doublet Three of the ordered sub-spectra coincided with a set of hyperfine sub-spectra of

α”-Fe16N2;15the sub-spectrum with a higher hyperfine field of 500 kOe is attributed to the Fe oxides

FIG 1 Magnetization curves of the α-Fe 16 N 2 aligned sample The external magnetic field was applied with an angle from the magnetic alignment direction The angle ranged from 0 to 90 ◦ and coercive fields decreased monotonically with an increase

of the angle Data were partly shown in a previous report.13

FIG 2 Mossbauer spectrum for the α-Fe 16 N 2 aligned sample recorded at 4.2 K Solid lines show the results of the fit Corresponding peak positions are indicated by three bars for magnetic subspectra of the α”-Fe 16 N 2 core and two bars, magnetic sextet (M), and paramagnetic doublet (D), for the outer shell of Fe oxides (Fe-O).

The fitting parameters at 4.2 K are listed in TableI From this fitting, these oxides were paramagnetic

at room temperature10and the corresponding subspectrum had an area ratio of 43.8% and an isomer shift of 0.38 mm/s, supporting that Fe atoms in the Fe-O shell had an Fe3+state.13Fe atoms belonging

to α”-Fe16N2 were found at a concentration of 54.4%, slightly less than that at room temperature, 56.2%.10,13

Consider that the sample consists of only Fe16N2and Fe2O3and 1 g of the whole sample contains

x mol of FeN1/8(1 mol = 57.6 g) and y mol of Fe-O3/2(1 mol = 80.0 g), respectively The equations 57.6x + 80.0y = 1 and x/y = 54.4/46.5 can be applied in this case Solving these equations gives

x = 0.00794; therefore, 1 g of the whole sample contains 0.457 g of FeN1/8 From this result,

Ms= 108 emu/g of the whole sample can be converted to 234 emu/g, namely 1750 emu/cm3 If we consider Fe3O4as Fe-O, instead of Fe2O3for reference, 232 emu/g was obtained and the difference

is less than 2% and relatively small

Magnetic torque curves were measured under a range of external magnetic fields lower than

10 kOe, shown in Fig 3 The torque curve under magnetic fields lower than the coercive field of 3.5 kOe showed almost two-fold symmetry (Fig 3 (a)), which changed to four-fold symmetry as the magnetic field was increased (Fig.3 (b)–(d)) The rotational hysteresis almost disappeared in the torque curve at 8.0 kOe, though the curve still deviated from a simple sinusoidal curve

The torque curves for the aligned α”-Fe16N2 NPs clearly show hysteresis loss at the middle range of the external magnetic field due to the hysteretic feature of magnetization curves, and they are plotted against the amplitude of the external field (Fig.4) Peaks in the curves were found at around 4 kOe for both aligned and random samples The hysteresis loss was plotted against the inverse of the magnetic field (Inset of Fig.4) The anisotropy field (Hk) was estimated from these plots using linear fits of straight lines,16as shown in the inset of Fig.4 The estimated amplitude of Hk

was 8.0 kOe for both samples From the relationship Ku= MsHk/2 (using the Stoner-Wohlfarth (SW)

model), Kuof α”-Fe16N2was calculated as 6.9 Merg/cm3 It should be noted that Hkdid not vary with

TABLE I Mossbauer parameters, hyperfine field (Hhf ), isomer shift (I.S.), quadrupole split (Q.S.), and area ratio for α”-Fe 16 N 2 aligned nanoparticles at 4.2 K.

Fe 16 N 2

a

056212-4 Kita et al. AIP Advances 7, 056212 (2017)

FIG 3 Typical magnetic torque cures for the aligned α”-Fe 16 N 2 particles sample (M2) at an external field of (a) 2 kOe, (b) 4 kOe, (c) 6 kOe, and (d) 8 kOe Rotational hysteresis was clearly observed in the curves below 6 kOe.

different degrees of alignment The hysteresis loss has been studied using numerical simulation for the coherent rotation (SW) and the funning models For both aligned and random samples, the peak positions of the hysteresis loss against the external field did not change significantly, although the peak height remarkably decreased for the random samples.17The peak found in the present study was not sharp compared with the theoretically calculated one,13and distribution in magnetic anisotropy was suggested The difference in peak height between the aligned and random samples qualitatively agreed with the theoretical simulation

FIG 4 Rotational magnetic hysteresis loss energy (H.L.) calculated from the torque measurements for α”-Fe 16 N 2 nanoparti-cles Closed “red” circles and open “blue” circles show data for the magnetically aligned sample and the non-aligned (random) sample, respectively In the Inset, the hysteresis losses were plotted against the inverse of the external magnetic field The anisotropy field (H ) was found to be 8.0 kOe where the linear line intersects with the abscissa.

The magnetic anisotropy of α”-Fe16N2has been studied for various samples, including NPs10,12,13 and thin films.7 Ogawa reported the performance of sub-micron sized particles with a diameter of 0.2 µm whose Mossbauer spectra indicate the particles are in a single-phase state;12the anisotropy energy amplitude from their study was reported as 9.6 Merg/cm3 From the size of the particles, the values is considered to be that for the bulk state Our results were obtained using NPs with 20 nm diameters, much smaller than those used in the study of Ogawa

The assignment of phases for magnetic subspectra in Mossbauer spectroscopy usually includes

a 2-3% error due to insufficient statistics The intrinsic uncertainty related to the recoil free ratio can

be lowered by recording the spectrum at the lowest temperature, eliminating the influence of recoil absorption The sample where the ratio of Fe atoms in α”-Fe16N2 compared to the whole sample changes from 56.2 % at room temperature to 54.4% at 4.2 K shows a relatively small temperature dependence The valence state of Fe atoms in the shell was mostly found to be 3+; however, the structure of shell Fe oxides was not well determined and might have a spinel-like structure with a small amount of Fe2+atoms This leads to an uncertainty in the volume and weight of the oxides in the sample and a corresponding error in the calculation of the saturation magnetization as large as

2 % Therefore, we estimated the maximum error to be 5 % in resulted magnetization and anisotropy Other techniques have been examined for the determination of phase concentration X-ray and neutron diffraction have been used for bulk-like samples with high resolution achieved by the Rietveld refinement An advantage of the latter18was pointed out in that there is less influence from penetration depth compared with the former technique In the present case, the particle size is small and the presence of the amorphous phase was confirmed by TEM In such cases, Mossbauer spectroscopy is meaningful in spite of the rather high estimated error of 5%

In summary, the magnetic anisotropy energy for core–shell α”-Fe16N2 NPs was evaluated by

the rotational hysteresis loss measured from torque measurements The saturation magnetization and anisotropy constant were evaluated to be 234 emu/cc and 6.9 Merg/cm3, respectively These amplitudes coincide well with those for bulk-like single phase α”-Fe16N2particles This crystalline

anisotropy is still smaller than the shape anisotropy of the thin films (2πMs2= 20 Merg/cm3) and a perpendicular magnetic state is not expected for the thin film form The characteristics is suitable for applications such as magnetic recording media9and bulk permanent magnets.13

ACKNOWLEDGMENTS

This work was supported by a grant-in-aid for scientific research under Grant 16H03190, received from The Ministry of Education, Culture, Sports, Science, and Technology of Japan The authors would like to express their thanks to Dr Minagawa for his help in the experiments M¨ossbauer studies were performed at the Tandem Accelerator Complex, University of Tsukuba

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